The present invention relates to a permanent magnet of R-TM-B system in which an electroplated copper layer having a fine crystal grain size is provided on a magnetic body to remarkably improve corrosion resistance property.
With higher performance and smaller size of electric and electronic equipment, similar demands have become increasingly great for permanent magnets used as parts of those equipment. More specifically, while the permanent magnet which seemed to be strongest in the past was made of rare-earth-element/cobalt (R-Co) system, a stronger permanent magnet of R-TM-B system has been recently put into practice (see JP-A-59-46008). Herein, R is one or more of rare earth elements including Y (yttrium), and TM is one or more of transition metals including typical elements of Fe and Co, a part of which may be replaced by any other metal element or nonmetal element. B is boron.
However, such a permanent magnet of R-TM-B system has suffered from the problem that the magnet is very likely to corrode. For this reason, it has been proposed to provide an oxidation-resistant protective layer on the surface of a permanent magnet body for improving corrosion resistance.
The proposed types of protective layer include an electroplated nickel layer, an oxidation-resistant resin layer, an aluminum ion-plated layer, and so forth. Above all, the nickel electroplating has drawn an attention because it is simple treatment and effective in improving corrosion resistance of the permanent magnet of R-TM-B system (see JP-A-60-54406). As compared with the method of using oxidation-resistant resin, the nickel electroplating is advantageous in that the resulting surface protective layer is excellent in mechanical strength and the layer will not in itself appreciably absorb humidity.
The nickel electroplating method, however, has a tendency that since the plating current is liable to concentrate on outer peripheral portions, such as corners, of the magnet body, the film thickness becomes relatively thick in those outer peripheral portions, while since the plating current is hard to pass through an inner hole and inner peripheral portions, the film thickness becomes relatively thin in those inner hole and inner peripheral portions. Accordingly, a sufficient degree of uniformity in the film thickness cannot be achieved by the nickel electroplating alone. For those magnets having a peculiar shape such as cylindrical magnets, in particular, there has arisen a problem that the electroplated nickel layer is hardly coated on the inner peripheral portions.
To solve the above problem of undesired uniformity in the film thickness, a method of providing an electroplated copper layer as an underlayer for the nickel electroplating has been proposed so far (see JP-A62-236345 and JP-A-64-42805, for example).
A plating bath which can be used in practice includes a cyanic copper bath and an alkaline organic acid salt of copper bath containing phosphoric ester as a primary ingredient. These baths are advantageous in that plating can be directly applied onto the surface of the permanent magnet of R-TM-B system, because they have no substitution action of copper.
The term "substitution action" used herein implies that when some metal at a lower-level position in the electrochemical series is immersed in a salt solution of another metal at a higher-level position in the electrochemical series than the above metal, the immersed metal is dissolved and the metal in the solution is instead reduced from an ionized state so that it is deposited to form a coating. For example, those metals which are at higher-level positions than neodymium and iron in the electrochemical series include chromium, 18-8 stainless steel (in activated state), lead tin, nickel (in activated state), brass, copper, bronze, Cu-Ni alloy, nickel (in passive state), 18-8 stainless steel (in passive state), silver, gold, platina, etc. Any appropriate one of those metals has been selected depending on demand.
Also, bright plating has been conventionally used for the reason that pin holes are few and corrosion resistance is superior. The term "bright" used herein means a state that the surface has microscopic smoothness. To obtain a bright surface, it has been conventionally practiced to select an optimum brightener in view of such factors as residual stress and hardness of the coating, or to slowly effect an electrolytic reaction with the so-called bright current density.
Regardless of whether being electrolytic or nonelectrolytic, however, the conventional copper plating has a disadvantage that the plated layer is easy to change color in air and is likely to cause surface oxidation. In other words, the electroplated nickel layer provided on the plated copper layer is a coating which is indispensable in maintaining corrosion resistance. But, the electroplated copper layer resulted from using a cyanic copper bath and the alkaline organic acid salt-of-copper bath containing phosphoric ester as a primary ingredient is formed as a film which has the surface configuration of a cellar structure that includes almost circular cells having the size of 10 to 50 .mu.m as shown in a photograph of FIG. 13, and also has somewhat rough structure with the crystal grain size of 0.5 to 2 .mu.m as shown in a photograph of FIG. 14. Particularly, in FIG. 14, there appears a sharp crack extending laterally from the upper left portion. Note that the photographs were taken at 500 magnifications for FIG. 13 and 10,000 magnifications for FIG. 14.
Thus, since the plated copper layer is formed as a film of cellar structure having such surface roughness, even if the plated nickel layer is coated on the plated copper underlayer, the resulting film is formed to exhibit the surface configuration of cellar structure having the surface roughness of 1 to 5 .mu.m as shown in a photograph of FIG. 15. This has raised the problem that pin holes remain in the plated nickel layer at the boundary portion of cellar structure and corrosion resistance is deteriorated. An attempt of avoiding a detrimental effect of the pin holes in the above case leads to another problem that the film thickness must be increased. In this connection, a laser microscope is to measure unevenness of the surface while scanning a laser beam at a location indicated by the center line in FIG. 15. Referring to FIG. 15, the uneven profile curve is present between an upper broken line, as a base, representing zero .mu.m and a lower broken line representing 5.28 .mu.m. The average depth (DEPTH) is also indicated by an arithmetic unit incorporated in the laser microscope. In the case of FIG. 15, DEPTH is 4.72 .mu.m.
Further, the bright plating has suffered from the problem that an optimum brightener must be selected depending on cases, or that such a range of bright current density as expending an inconvenient amount of time must be selected at the sacrifice of productivity. Additionally, because brighteners contain sulfur (S), there is another problem that if due consideration is not paid to the relationship between a brightener used and an underlying or overlying layer, an electrochemical local battery may be formed to reduce corrosion resistance against the intention.